The update of the share of a cfs_rq is done when its load_avg is updated
but before the group_entity's load_avg has been updated for the past time
slot. This generates wrong load_avg accounting which can be significant
when small tasks are involved in the scheduling.
Let take the example of a task TA that is dequeued of its task group TG1.
TA was the only task in TG1 which becomes idle.
We have the sequence:
- dequeue_entity TA->se
- update_load_avg(TA->se)
- dequeue_entity_load_avg(TG1->cfs_rq, TA->se)
- account_entity_dequeue(TG1->cfs_rq, TA->se)
TG1->cfs_rq->load.weight = 0
- update_cfs_shares(TG1->cfs_rq)
TG1->se->load.weight is updated with the new share of
cfs_rq. TG1->se->load.weight = 0.
- dequeue_entity TG1->se
- update_load_avg(TG1->se) but its weight is now null so the last time
slot (up to a tick) will be accounted with its new weight (0 in our case)
instead of its real weight. The last time slot is accounted as an idle one
whereas it was a running one.
If the running time of TA is short enough that no tick happens when it
runs, all running time of TG1->se will be accounted as idle time.
Instead, we should update the share of a cfs_rq (in fact the weight of its
group entity) only after having updated the load_avg of the group_entity.
update_cfs_shares() now takes the sched_entity as parameter instead of the
cfs_rq and the weight of the group_entity is updated only once its load_avg
has been synced with current time.
Cc: <sta...@vger.kernel.org>
Signed-off-by: Vincent Guittot <vincent.guit...@linaro.org>
---
I have seen the problem on tip/sched/core, v4.8 and v4.7. Previous versions
might also have the problem but I haven't not been able to test them yet.
kernel/sched/fair.c | 27 ++++++++++++++++-----------
1 file changed, 16 insertions(+), 11 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 18d9e75..19092fa 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2689,15 +2689,18 @@ static void reweight_entity(struct cfs_rq *cfs_rq,
struct sched_entity *se,
static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
-static void update_cfs_shares(struct cfs_rq *cfs_rq)
+static void update_cfs_shares(struct sched_entity *se)
{
struct task_group *tg;
- struct sched_entity *se;
+ struct cfs_rq *cfs_rq = group_cfs_rq(se);
long shares;
+ if (entity_is_task(se))
+ return;
+
tg = cfs_rq->tg;
- se = tg->se[cpu_of(rq_of(cfs_rq))];
- if (!se || throttled_hierarchy(cfs_rq))
+
+ if (throttled_hierarchy(cfs_rq))
return;
#ifndef CONFIG_SMP
if (likely(se->load.weight == tg->shares))
@@ -2707,8 +2710,10 @@ static void update_cfs_shares(struct cfs_rq *cfs_rq)
reweight_entity(cfs_rq_of(se), se, shares);
}
+
+
#else /* CONFIG_FAIR_GROUP_SCHED */
-static inline void update_cfs_shares(struct cfs_rq *cfs_rq)
+static inline void update_cfs_shares(struct sched_entity *se)
{
}
#endif /* CONFIG_FAIR_GROUP_SCHED */
@@ -3583,9 +3588,9 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity
*se, int flags)
se->vruntime += cfs_rq->min_vruntime;
update_load_avg(se, UPDATE_TG);
+ update_cfs_shares(se);
enqueue_entity_load_avg(cfs_rq, se);
account_entity_enqueue(cfs_rq, se);
- update_cfs_shares(cfs_rq);
if (flags & ENQUEUE_WAKEUP)
place_entity(cfs_rq, se, 0);
@@ -3681,7 +3686,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity
*se, int flags)
/* return excess runtime on last dequeue */
return_cfs_rq_runtime(cfs_rq);
- update_cfs_shares(cfs_rq);
+ update_cfs_shares(se);
/*
* Now advance min_vruntime if @se was the entity holding it back,
@@ -3864,7 +3869,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity
*curr, int queued)
* Ensure that runnable average is periodically updated.
*/
update_load_avg(curr, UPDATE_TG);
- update_cfs_shares(cfs_rq);
+ update_cfs_shares(curr);
#ifdef CONFIG_SCHED_HRTICK
/*
@@ -4761,7 +4766,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p,
int flags)
break;
update_load_avg(se, UPDATE_TG);
- update_cfs_shares(cfs_rq);
+ update_cfs_shares(se);
}
if (!se)
@@ -4820,7 +4825,7 @@ static void dequeue_task_fair(struct rq *rq, struct
task_struct *p, int flags)
break;
update_load_avg(se, UPDATE_TG);
- update_cfs_shares(cfs_rq);
+ update_cfs_shares(se);
}
if (!se)
@@ -9316,7 +9321,7 @@ int sched_group_set_shares(struct task_group *tg,
unsigned long shares)
/* Possible calls to update_curr() need rq clock */
update_rq_clock(rq);
for_each_sched_entity(se)
- update_cfs_shares(group_cfs_rq(se));
+ update_cfs_shares(se);
raw_spin_unlock_irqrestore(&rq->lock, flags);
}
--
2.7.4
